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Elizabeth Clem, a member of the
Wireline Reflector Mailing List, submitted a section of a cement bond log to the group for comment (the log section is reproduced below). She was seeking an explanation for the odd repeating anomaly observable in the CCL record. A number of theories were put forward, but the folks at Penkota Wireline Services were the first to say they had seen something similar where the anomalies were caused by magnetic lifting equipment used to load casing. While a few responders remained skeptical, inquiries by Ms. Clem to the mill ultimately confirmed the magnetized pipe theory. A photograph of the magnetic lifting crane is shown below, and information supplied by the mill confirms the magnet spacing coincides with the repeating anomalies.
Note that the "real" collars exhibit the expected decrease in amplitude in the free (uncemented) pipe log section below, the expected slight increase in travel time, and the characteristic "chevron" or "W" signature in the VDL track. The magnetic anomalies do not produce any acoustic changes in the sonic log curves. It is also noteworthy that there are no changes in line tension at the anomalies. The magnetic anomalies are physically longer than the collars as evidenced by the longer event time / lower frequency of the anomalies versus the true collars.
Ray Ethridge with Chappel Hill Logging Systems, arguably the world's leading expert on neutron generators and related logging technology, proposed on the
Wireline Reflector Mailing List that since it is an industry convention to name an effect in honor of its discoverer, henceforth a magnetically induced anomaly on a CCL log will be due to the Clem Effect. The proper way to mark them on a log would be to circle them and write "Clems" by the side. Ray went on to say that one could then reference this webpage and that should satisfy even the most critical company man, right? Sounds reasonable and amusing to us...
If you see weird repeating CCL anomalies, magnetic lifting equipment that has inadvertently magnetized the pipe placed in the well should be eliminated as a possible cause before more exotic explanations are explored.
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